Many might think Luffy is surprised at Lokis strength. I don't think that is it, Luffy is incredible at sensing other peoples strength and their intentions/nature.
What I do think happened is Luffys battle IQ kicking in. No other character in the series have consistently showed that they can learn techniques just by observing for a short while. Be it Crocodiles weakness, CP9s Soru, Katakuris FS or Kaidos CoC-attacks. Heck, you could probably go all the way back to Captain Morgan and Kuro and make a case for early CoO.
So what I think happened is Luffy realizing what Gaban could not - Lokis attacks probably merge Ryou (internal) with CoC or something like that. I don't think it is the DF or Ragnir but Loki by chance has something similar to Hyogoros Ryou, no immortal body can protect you from an attack that crushed your soul.
Apart from Luffy we know another character who achieved the impossible by observing - Shanks stealing the Kamusari move. I think he also learned this way of attack from Loki. And by the time he learned the same way of attacking he could surpass Loki/level the playing field because he is likely more adept as a swordsman and CoC even though Loki looks to be his match at CoO and greater body.
Shanks having access to this type of attack is probably exactly why the Gorosei said he is beyond their reach. He can actually destroy them.